Wave soldering tank

ABSTRACT

A wave soldering tank includes a soldering tank body for housing molten solder and a solder feed chamber disposed within the soldering tank body. An axial-flow, multiple-blade screw-type pump is disposed so as to draw molten solder into the solder feed chamber through an inlet and discharge the molten solder through an outlet. In a preferred embodiment, the pump includes a rotatable hub and a plurality of helical blades secured to the hub at equal intervals in the circumferential direction of the hub, each of the blades overlapping an adjoining one of the blades when the blades are viewed in the axial direction of the impeller.

TECHNICAL FIELD

This invention relates to a wave soldering tank having a pump forsupplying molten solder within the tank to a nozzle.

BACKGROUND ART

A wave soldering tank typically includes a pump submerged within moltensolder in the tank. When the pump is operated, molten solder is suckedinto an inlet of the pump and then discharged from a nozzlecommunicating with an outlet of the pump. By suitably controlling thepump, the solder discharged from the nozzle can be formed into a wavethrough which electronic parts can be passed for soldering.

FIG. 1 is a schematic plan view of a conventional wave soldering tankdisclosed in Japanese Published Unexamined Patent Application2003-136233. As shown in this figure, a solder feed chamber 94 which isdisposed in a soldering tank 90 includes a casing 92 in which amultiple-blade centrifugal pump 91 (such as a sirocco pump) is provided.The solder feed chamber 94 communicates with a nozzle 93 from whichmolten solder can be discharged.

With this conventional apparatus, only approximately ¼ of the entireperiphery of the casing 92 is open for molten solder to be dischargedtherefrom to the solder feed chamber 94. At point A in FIG. 1, themolten solder discharged from the pump 91 is traveling approximatelytangentially with respect to the pump 91. Point B is spaced from point Aby approximately ¼ of the distance around the pump 91. There is a bigdifference in the discharge speed of molten solder between the vicinityof point A and the vicinity of point B, and this speed difference mayproduce undesirable undulations in solder which is discharged from thenozzle 93. In order to minimize the undulations, flow straighteningplates (not shown) are commonly provided in a duct 94 leading from thecasing 92.

DISCLOSURE OF THE INVENTION

In recent years, in order to increase productivity, there has been ademand for an increase in the speed of wave soldering machines, for theability to perform wave soldering on a wider variety of parts, and forthe ability to perform wave soldering on parts which in the past havebeen considered to be difficult to solder.

The present inventors found that such demands can be met by improvingthe structure of a soldering tank.

However, with the above-described pump shown in FIG. 1, molten solder isdischarged from only approximately ¼ of the entire periphery of thepump, so the efficiency of the pump is poor. In addition, if flowstraightening plates are provided to reduce undulations, oxidized drossadheres to and aggregate on the plates and leave the plates. Thus solderwhich is discharged from the nozzle becomes polluted. In addition, it isdifficult to suppress undulations even with the provision of flowstraightening plates.

Thus, one object of the present invention is to provide a wave solderingtank which can supply molten solder to a nozzle with good efficiency,which can eliminate undulations in solder discharged from the nozzle,and which can prevent oxidized coarse dross from being mixed into solderdischarged from the nozzle.

Another object of the present invention is to provide a soldering tankwhich can more smoothly transport molten solder within a solder feedchamber and which can pressurize the interior of the solder feed chamberwithout the occurrence of turbulence.

The present invention provides a wave soldering tank comprising asoldering tank body for housing molten solder and a solder feed chamberdisposed within the soldering tank body. The solder feed chamber has aninlet disposed below the level of molten solder in the soldering tankbody and an outlet disposed above the level of molten solder in thesoldering tank body. An axial-flow, multiple-blade screw-type pump isdisposed in the soldering tank body so as to draw molten solder into thesolder feed chamber through the inlet and discharge the molten solderthrough the outlet.

In preferred embodiments, the pump includes an impeller having arotatable hub and a plurality of helical blades secured to the hub atequal intervals in the circumferential direction of the hub. Each of theblades overlaps an adjoining one of the blades when the blades areviewed in the axial direction of the impeller. The hub may be a cylinderor a solid shaft.

In a wave soldering tank according to the present invention, the pump isan axial-flow pump, so solder does not flow radially outwards from thepump but is transported in the axial direction of the pump. As a result,pressure is efficiently and uniformly applied to the interior of thesolder feed chamber. If the rotation of the pump causes solder to besent straight downwards, i.e., towards the bottom surface of the tank,when the bottom surface is horizontal, the solder is reflected and risesimmediately beneath the pump. However, because the helical bladesoverlap each other as viewed in the axial direction of the impeller,solder cannot pass in a straight line through the pump, so solder isprevented from rising towards the pump. As a result, the pressure withinthe solder feed chamber can be uniformly increased without turbulence.“Each of the blades overlaps an adjoining one of the blades when theblades are viewed in the axial direction of the impeller” means thatwhen, for example, the pump impeller has two helical blades spaced fromeach other around the hub by 180°, each helical blade spirals by atleast 180° around the hub between the first and second ends of theblade. When the pump impeller has three helical blades disposed atintervals of 120°, each helical blade spirals around the hub by at least120° between its first and second ends. This is the same for the casewherein four helical blades are provided. Thus, if the pump impellerincludes N blades disposed at intervals of 360/N degrees around the hub,each blade spirals around the hub by at least 360/N degrees between itsfirst and second ends.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a conventional wave soldering tank.

FIG. 2( a) is a front cross-sectional elevation of an embodiment of awave soldering tank according to the present invention, and FIG. 2( b)is a side cross-sectional elevation thereof as viewed from the right inFIG. 2( a).

FIG. 3 is a cutaway perspective view of the pump of the embodiment ofFIGS. 2( a) and 2(b).

FIG. 4( a) is a plan view of the impeller of the pump of FIG. 3, andFIG. 4( b) is an elevation of the impeller.

FIG. 5 is a front cross-sectional view of another embodiment of a wavesoldering tank according to the present invention.

FIG. 6( a) is a bottom plan view of a pump used in the presentinvention, and FIG. 6( b) is a bottom view of a conventional pump for awave soldering tank.

FIG. 7 is a front view of another embodiment of a pump used in thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The structure of a wave soldering tank according to the presentinvention will be described in greater detail while referring to theaccompanying drawings.

As shown in FIGS. 2( a) and 2(b), which are cross-sectional elevationsof a first embodiment of a wave soldering tank according to the presentinvention, this embodiment includes a soldering tank body 1 which isopen at its upper end and a solder feed chamber 2 disposed in the tankbody 1.

The solder feed chamber 2 has an inlet 3 which is disposed lower thanthe liquid level L and an outlet 4 which is disposed higher than theliquid level L of molten solder in the tank body 1. A pump 5 isinstalled at the inlet 3.

In this embodiment, the solder feed chamber 2 includes a partition 6which is disposed beneath the liquid level L in the tank body 1 anddivides the interior of the tank body 1 into an upper and lower portion.The inlet 3 comprises a through hole formed in the partition 6. Anotherthrough hole 7 which communicates with the outlet 4 is formed in thepartition 6 in a location spaced from the inlet 3. A duct 8 is securedto and extends upwards from the partition 6 at through hole 7. The upperend of the duct 8 is partially closed off by a lid 9 having a throughhole 10 with an area smaller than the horizontal cross-sectional area ofthe duct 8. A nozzle 11 is installed in the through hole 10 and extendsupwards from the lid 9 to above the liquid level L in the tank body 1.The outlet 4 of the solder feed chamber 2 comprises the upper end of thenozzle 11, from which molten solder can be discharged to form a wave.

The solder feed chamber 2 may be an independent structure which isinstalled on the bottom of the soldering tank body 1, but taking intoconsideration the buoyancy of molten solder, the above-describedstructure using a partition 6 is simpler and therefore preferable.

As shown in detail in FIGS. 3, 4(a), and 4(b), the axial-flow pump 5used in the present embodiment includes a cylindrical casing 12 having acylindrical interior 13, and a multiple-blade screw-shaped impeller 14disposed in the casing 12 for rotation about its longitudinal axisinside the casing 12. The impeller 14 shown in the Figures has fourblades. The impeller 14 may have two or more blades 21. Preferably ithas at least four blades 21.

The length of the casing 12 is usually such as to surround the impeller14 over its entire length. Therefore, the length of the casing 12 may bethe same as or a little shorter than the overall length of the impeller14. Preferably, the end of the impeller 14 extends 5-10 mm ahead of theend of the casing so that the molten solder can be smoothly anduniformly supplied to the solder feed chamber 2.

The impeller 14 may be rotated about its axis by any suitable drivemechanism. In the present embodiment, as shown in FIGS. 2( a) and 2(b),the impeller 14 is secured to the lower end of a drive shaft 18 which isrotatably supported by a bearing 19. The drive shaft 18 can be rotatedby an electric motor 15 which is drivingly connected to the drive shaft18 by gears 16 and 17. The drive mechanism including the motor 15 andthe gears 16 and 17 may be supported in any suitable manner, such as byan unillustrated support secured to the tank body 1 or the partition 6.

As best shown in FIG. 3, the illustrated impeller 14 includes acylindrical hub 20 and a plurality of helical blades 21 (four in thisembodiment) mounted on the hub 20 at equal intervals around thecircumference of the hub 20. The upper and lower end surfaces 28 of theblades 21 are preferably flush with the upper and lower surfaces of thehub 20.

Each of the helical blades 21 extends helically around the hub 20between the first and second lengthwise ends of the hub 20. The angle ofspiral, i.e., the angle between the first and second ends of each blade21 as measured from the center of the hub 20 is such that when theimpeller 14 is viewed in its axial direction, each of the blades 21overlaps an adjoining one of the blades 21 in the circumferentialdirection of the impeller 14. When the impeller 14 includes four equallyspaced blades 21, the angle of spiral is at least 90°, preferably atleast 120°, and ideally at least 180°. As shown in FIG. 4( a), in theillustrated embodiment, the angle of spiral of each blade 21 is 210°.The smaller the angle of slope α (shown in FIG. 4( b)) of the blades 21with respect to a plane perpendicular to the axis of the impeller 14,the more easily pressure can be applied to molten solder in the solderfeed chamber 2, so the angle of slope α is preferably at most 45°.

The impeller 14 may be secured to the drive shaft 18 in any convenientmanner. In the illustrated embodiment, the hub 20 (FIG. 3) fits over thelower end of the drive shaft 18, with the upper end of the hub 20pressed against a step portion 22 formed on the drive shaft 18 and thelower end of the hub 20 pressed by a flange 23 secured to the lower endof the drive shaft 18 beneath the hub 20. Thus, as shown in FIGS. 2( a)and 2(b), the hub 20 is sandwiched from above and below.

FIG. 5 illustrates another embodiment of a wave soldering tank accordingto the present invention. The same parts as in FIGS. 2( a) and 2(b) areindicated by the same reference numbers. In this embodiment, adish-shaped guide 25 for smoothly guiding solder between the inlet 3 andthe outlet 4 is secured to the lower side of the partition 6. Theportions of the guide 25 immediately below the inlet 3 and the throughhole 7 in the partition 6 have curved surfaces 26 and 27, such assurfaces described by an arc of a circle. Solder which is discharged bythe pump 5 proceeds straight downwards from the pump 5 and then strikescurved surface 26 and is led in the horizontal direction. The solderthen strikes against the other curved surface 27 and is led straightupwards. In this manner, the efficiency of transport of molten solder inthe solder feed chamber 2 is improved. The structure of this embodimentis otherwise the same as that of the preceding embodiment.

According to the present invention when the pump 5 is driven, moltensolder in the tank is sucked through the upper end of the casing 12 andis then discharged downwards through four blades into the lower end ofthe casing 12, i.e., into the solder feed chamber 2. As the impeller 14rotates, the location where the solder is discharged by the impeller 14also rotates, so by uniformly discharging solder along the entireperiphery of the lower end of the pump 5 except for the regionimmediately beneath the hub 20, the efficiency of the pump 5 isincreased, and the pressure applied to the interior of the solder feedchamber 2 is the same in any location.

Accordingly, almost no undulations occur in the solder which isdischarged from the outlet 4. If the rotational speed of the impeller 14is maintained constant, the height of the solder which is dischargedfrom the outlet 4 can be always maintained constant. Alternatively, byadjusting the rotational speed of the impeller 14, the height of thesolder within the outlet 4 can be adjusted. Thus, the height of themolten solder in the outlet 4 can be adjusted by controlling therotational speed of the impeller 14.

FIG. 6( a) and FIG. 6( b) are bottom plan views of a pump used in thepresent invention, i.e., a pump 5 having a impeller 14 having aplurality of helical blades 21, and of a comparative example of a pump95, i.e., a pump using a impeller 14 having a single helical blade 21,respectively.

It is thought that with a screw-shaped impeller, solder is mostefficiently discharged from the vicinity of the lower end surface 28 ofeach helical blade 21. See FIG. 6( b). Therefore, in the case of a pump95 having a single helical blade 21, solder is thought to be dischargedfrom a single location. When the impeller 14 is rotating at a low speed,the location from which solder is discharged slowly rotates by 360° inthe circumferential direction of the impeller, and this producesundulations in the discharged solder. In order to prevent suchundulations and discharge solder uniformly around the entirecircumference of the lower end of the impeller 14, it is necessary torotate the impeller 14 at a high speed. However, in order to achieve ahigh speed of rotation, it is necessary to markedly increase thestrength of the impeller 14 itself in order to prevent breakage thereof.In addition, a high speed of rotation results in a large amount ofsolder being discharged from the impeller 14, and it becomes difficultto perform fine adjustment of the height of the solder which isdischarged from the outlet.

In contrast, with a plurality of helical blades 21 disposed at equalintervals as in the present invention, as can be seen from FIG. 6( a),solder is charged from a plurality of locations around the circumferenceof the impeller, and the locations from which solder is discharged arewell balanced in the circumferential direction, so even at a lowerrotational speed than in the comparative example, solder can bedischarged uniformly around the circumference of the lower end of theimpeller. Because a low rotational speed can be used, fine adjustment ofthe height of the solder which is discharged from the outlet can beeasily carried out.

In the case of the wave soldering tank shown in FIGS. 2( a) and 2(b), ifsolder is discharged directly downwards by the pump 5, due to reflectionof the solder off the bottom surface of the tank body 1, a directlyupward force is applied which prevents solder from being transported bythe lower bottom surface of the soldering tank body 1. However, it isthought that this force is effectively suppressed by the helical blades21 and particularly by the lower end surface 28 of the blades 21. Thus,by providing a plurality of helical blades 21 and installing them atequal intervals, an upward force exerted by reflected solder can besuppressed markedly more efficiently than in the comparative example.

FIG. 7 illustrates an arrangement of the casing and the pump, in whichin a preferred embodiment the distance T, i.e., the length by which theimpeller extends beyond the end of the casing is defined as 5-10 mm sothat the molten solder can be charged uniformly to the solder feedchamber 2. Furthermore, according to another preferred embodiment,clearance C between the inner wall of the casing 12 and the screw, i.e.,helical blades are defined as 0.1-1 mm so as to supply molten solderuniformly without formation of pulsating flow.

INDUSTRIAL APPLICABILITY

In a wave soldering tank according to the present invention, moltensolder is uniformly discharged from the lower end of a pump over theentirety of the bottom surface of the pump except for the regionimmediately beneath the hub of he pump, so compared to a wave solderingtank using a conventional pump in which molten solder is discharged froma region of only ¼ of the entire periphery of the pump, the efficiencyof transport of solder is improved. In addition, molten solder isuniformly discharged from most of the entire bottom surface of the pump,so turbulence is eliminated, and the pressure applied to the solder feedchamber becomes the same in any position. As a result, almost noundulations are observed in the flow of molten solder which isdischarged from an outlet through a nozzle without the need for flowstraightening plates. In addition, since flow straightening platesbecome unnecessary, oxidized dross is not introduced into solder whichis discharged from the outlet, so the cleanliness of solder isincreased.

1. A soldering tank comprising a soldering tank body for housing moltensolder, a horizontal partition extending across the tank body below thelevel of molten solder in the tank body and creating a barrier betweenan upper portion of the tank body located above the partition and alower portion of the tank body located below the partition and supportedby side walls of the tank body such that molten solder is prevented fromflowing from the upper portion to the lower portion of the tank bodybetween the side walls of the tank body and the outer periphery of thepartition around the entire periphery of the partition, the partitionhaving first and second openings horizontally spaced from each other, abowl-shaped guide secured to a lower side of the partition and havingcurved surfaces which are curved directly beneath the first and secondopenings for guiding fluid beneath the first and second openings, anozzle having a lower end in fluid communication with the second openingin the partition and an upper end disposed above the level of moltensolder in the tank body, and a multiple-blade screw-type pump having animpeller disposed so as to pump molten solder downwards through thefirst opening into a space between the partition and the bowl-shapedguide.
 2. A soldering tank as claimed in claim 1 wherein the impeller isdisposed in the first opening of the partition.
 3. A soldering tank asclaimed in claim 1 wherein the impeller includes at least four helicalblades.
 4. A soldering tank as claimed in claim 3, wherein each of theblades overlaps an adjoining one of the blades when the blades areviewed in the axial direction of the impeller.
 5. A soldering tank asclaimed in claim 4 wherein the blades are provided at equal intervals inthe circumferential direction of the impeller, each blade extending inthe circumferential direction by at least 120° between first and secondends of the blade.
 6. A soldering tank as claimed in claim 3 whereineach of the blades is sloped by at most 45° with respect to a planeperpendicular to a rotational axis of the impeller.
 7. A soldering tankas claimed in claim 1 including a duct extending upwards from thepartition above the second opening and communicating between the secondopening and the lower end of the nozzle.
 8. A soldering tank as claimedin claim 7 wherein there is no flow straightening plate in the solderingtank between the pump and an interior of the nozzle.
 9. A soldering tankas claimed in claim 1 wherein each curved surface is curved from thelower side of the partition to directly beneath one of the first andsecond openings.
 10. A soldering tank as claimed in claim 9 wherein eachof the curved surfaces comprises an end wall of the guide.
 11. Asoldering tank as claimed in claim 1 wherein the bowl-shaped guide isdisposed inside the tank body between the partition and a bottom innersurface of the tank body.
 12. A soldering tank as claimed in claim 1wherein the partition is a flat plate extending to a vicinity of theside walls of the tank body.
 13. A soldering tank as claimed in claim 1including a cylindrical casing disposed in the first opening of thepartition surrounding the impeller, the impeller being rotatablydisposed in the casing so as to transport molten solder in an axialdirection of the casing.
 14. A soldering tank as claimed in claim 13wherein a lower end of the impeller extends 5-10 mm below a lower end ofthe casing.
 15. A soldering tank as claimed in claim 13 wherein aclearance between the casing and the impeller is 0.1-1 mm.